1. Field of the Invention
The present invention relates to telecommunications and, more particularly, to telephone networks served by an external call server such as an IP Centrex server for instance.
2. Description of Related Art
Many enterprise phone systems today include a private branch exchange (PBX) server to support calling between telephone stations. In effect, a PBX server is a private version of a telephone company central switching office (exchange). Typically, the PBX server connects by respective telephone lines to all of the telephone stations in the system and by a PBX trunk to the local central office. The PBX server then allows connected telephone stations to call each other and to place and receive calls on the public switched telephone network (PSTN) via the central office. Further, a PBX server typically provides the telephone stations with enhanced calling features, such as abbreviated (e.g., extension) dialing, intercom service, call forwarding, call transfer, call restrictions, and conference calling.
As an alternative to PBX service, many local telephone companies now offer a “Centrex” service. In effect, Centrex is a single-line telephone service that provides each line in an enterprise with all of the “bells and whistles” commonly found in an enterprise PBX system. With Centrex, each enterprise telephone station is connected by a respective telephone line to the telephone company central office. There, a Centrex server provides each enterprise telephone line with the same sorts of enhanced calling features that a PBX server could provide, such as abbreviated dialing, intercom service, call forwarding, call transfer, call restrictions, and conference calling, for instance.
Centrex service has several advantages compared with PBX service. For one, Centrex service is typically cheaper to install than PBX service, because the central office switching infrastructure already exists. Further, a Centrex server can easily serve multiple locations of a given enterprise at once, whereas PBX service would require a dedicated line to tie together the various locations. And still further, Centrex service provides each enterprise telephone station with a direct dial (outside) line, whereas PBX service typically offers just a limited number of outside lines to be shared among all of the PBX extensions.
On the other hand, Centrex service has a significant downside, in that each telephone station needs to be connected by a respective telephone line with the central office. This means that each time someone new joins the enterprise, the enterprise must arrange with the telephone company to install telephone line to the new employee's desk. Unfortunately, this can take a lot of time and, in some scenarios, can be very expensive and difficult to accomplish.
To solve this problem, some carriers have begun to offer an improved version of Centrex service known as “IP Centrex.” In a typical IP Centrex system, an enterprise telephone network is coupled through one or more routers or gateways with an IP wide area network (WAN) (e.g., the Internet) on which an IP Centrex server resides, and the IP WAN is then coupled via one or more gateways with the PSTN. With this arrangement, packet-based call setup signaling will flow between the enterprise network and the IP Centrex server, to allow the IP Centrex server to set up calls between telephone stations within the enterprise, as well as calls between enterprise telephone stations and the PSTN.
In operation, for instance, when a user in the enterprise dials an extension of another user in the enterprise, the calling station (or a suitable gateway) may send a packet-based call-setup message (e.g., a “SIP” signaling message) via the IP WAN to the IP Centrex server, seeking to set up the call to the dialed extension. The IP Centrex server may then engage in packet-based signaling communication with the calling and called stations, to set up the call between the two stations. Similarly, when a user in the enterprise dials an outside number, the IP Centrex server may responsively set up the call over a bearer path that includes the enterprise network, the IP WAN, and the PSTN.
A significant benefit of IP Centrex service is that the enterprise only needs to have a single broadband connection with the IP WAN, rather than having to couple each telephone station by a respective line to the telephone company central office. However, with this benefit also comes a significant risk: if the enterprise network loses its connection with the IP WAN, such as if a critical router or link between the enterprise network and the IP WAN fails, the enterprise telephone system will no longer work. In particular, enterprise users will no longer bet able to set up calls via the PSTN. But even worse, enterprise users will no longer be able to set up calls with each other, since the entity that sets up their calls would be unreachable.